Within the disciplines of the clinical, industrial and life science laboratory, scientists perform methods and protocols with extremely small quantities of fluids. These fluids consist of many categories and types with various physical properties. Frequently, volumes are worked with that are between a drop (about 25 microliters) and a few nanoliters. There are a number of standard methods employed to transfer liquid compounds from a source by aspirating the liquid from such fluid holding devices into a fluid handling device having a probe, cannula, pin tool or other similar component or plurality of components that move, manually or robotically, and then dispensing, from the same probe or plurality of probes, into another fluid holding device.
Four common techniques are (1) a scheme using a probe or cannula, that may or may not be coated with a layer of material or special coating, which is attached directly or by a tube to a pumping device, (2) a scheme using a disposable pipette instead of the probe/cannula but otherwise similar, (3) a scheme using a spray head with one or a plurality of openings and pumping system that physically propels multiple precisely metered microdroplets, and (4) a scheme using metal shafts with precisely machined hollowed out spaces at their ends that hold the fluid by surface tension (commonly referred to as a “pin tool”).
As routine a process as fluid transfer is in the laboratory, technical challenges remain to achieve suitable levels of cleanliness of the dispensing devices. Currently the fluid handling devices undergo a “tip wash” process wherein they are cleansed in between use with a liquid solvent, such as DMSO or water. After the “tip wash” process, the used and now contaminated liquids must then be properly disposed of with respect to the required environmental regulations. As an alternative to this wet “tip wash” process, atmospheric pressure plasma can be used to replace the liquid cleaning process with a “dry” plasma cleaning process, thus eliminating the need for the handling and disposal of solvents that are biohazards and environmentally unfriendly.
The term “plasma” is generally used to denote the region in an electric gas discharge that has an equal number of positive ions and negative electrons (N. St. J. Braithwaite, “Introduction to gas discharges” Plasma Sources Science and Technology, V9, 2000, p517–527; H. Conrads et al., “Plasma Generation and Plasma Sources” Plasma Sources Science and Technology, V9, 2000, p441–454). A non-thermal, or non-equilibrium, plasma is one in which the temperature of the plasma electrons is higher than the temperature of the ionic and neutral species. Within an atmospheric pressure non-thermal plasma, there is typically an abundance of other energetic and reactive particles, such as ultraviolet photons, excited and/or metastable atoms and molecules, and free radicals. For example, within an air plasma, there are excited and metastable species of N2, N, O2, O, free radicals such as OH, NO, O, and O3, and ultraviolet photons ranging in wavelengths from 200 to 400 nanometers resulting from N2, NO, and OH emissions.
The “dry” plasma cleaning process is achieved by exposing the surfaces of the fluid handling devices or other components to the atmospheric pressure plasma. The above mentioned reactive and energetic components can now interact with any contaminants on the surfaces, thereby volatizing, dissociating, and reacting with the contaminants, to form smaller and benign gaseous compounds that are vented off through the plasma cleaning device.
In addition to removing various unwanted compounds, the plasma can also be used to sterilize the surfaces of the fluid handling devices. The same ultraviolet photons, especially those with wavelengths below 300 nm, the free radicals and metastable molecules, and the plasma electrons and ions, provide a very harsh environment in which bacteria, viruses, fungi and their corresponding spores are lysed or otherwise rendered non-viable and either partially or completely volatized into gaseous compounds.